Determining the performance and behavior of a telecommunication (telecom) network includes nonlinear characterization and modeling of components of the network. A component (or group of components) characterized using test signals from a measurement instrument is referred to as a device under test (DUT). A typical response of a DUT to a stimulus generated by a vector network analyzer (VNA) using a pair of tones, for example, includes a signal with a number of different frequency components including harmonics, intermodulation distortion products, modulation sidebands, etc. The frequency components are generated through the nonlinear behavior of the DUT. It is not possible to accurately determine the phase of all of these components using a VNA with conventional calibrations because the measurement hardware phase lengths are different at different frequencies.
Referring to FIG. 1, a harmonic phase standard 102 can be used with a VNA 10 to establish a phase reference for use in nonlinear measurements. A first port 12 of the VNA is connected with an input of the harmonic phase standard 101 while an output 103 of the harmonic phase standard is connected with a second port 14 of the VNA. The harmonic phase standard receives a signal from the VNA and introduces nonlinearities to the received signal and then provides the modified signal to the receiver of the VNA. The signal provided to the VNA can, for example, be stepped through a set of frequencies to characterize phase response to nonlinearities at different frequencies within the set of frequencies. The phase response is used to establish a phase reference.
A harmonic phase standard creates a waveform rich in signal content to act as a transfer standard so that the phase relationship of all of the components of that signal content can be determined. Knowledge of the phase relationship can then be transferred to a receiver of the VNA so that the VNA can accurately measure the phase of signal components coming from the DUT. The phase reference can be used in conjunction with load-pull systems, for example, and large-signal VNAs to characterize and model the nonlinear behavior of DUTs in a telecom network.
Traditional harmonic phase standards can be based on several different electronic components. For example, a traditional harmonic phase standard can be based on the use of a Schottky diode, as described in the article “Measurement of Magnitude and Phase of Harmonics Generated in Nonlinear Microwave Two-ports,” authored by U. Lott and published in the journal IEEE Transactions on Microwave Theory and Techniques, vol. 37, pp. 1506-1511, October 1989. However, it has been observed that the approach of using a Schottky diode can result in output that is limited in harmonic content.
In another approach, a traditional harmonic phase standard can be based on the use of a step-recovery diode, as described in the paper “Harmonic Generation using Step Recovery Diodes and SRD Modules”, Application Note 920, provided by Agilent Technologies. However, it has been observed that the approach of using a step-recovery diode suffers from phase stability issues.
In another approach, a traditional harmonic phase standard can be based on the use of an indium phosphide (InP)-based device such as a comb generator by Agilent Technologies. However, it has been observed that the approach of using an InP-based device provides high phase stability at the expense of limited frequency scalability due to circuit complexity. It can also suffer from degraded phase flatness with frequency.